Many surface finishing processes, particularly repair processes in the automotive refinishing industry and marine industry, involve the application of fillers to imperfections in the surface followed by progressive abrasion with coarser to finer abrasive materials until the desired smooth surface is achieved. A series of protective coatings is applied, e.g., primer, paint coats, lacquers and so forth, and each layer is normally rubbed down prior to application of the next coat in order to ensure a smooth even surface.
In order to assist the operator in identifying areas requiring abrasion and highlighting defects during the repair process, the operator applies a guide coat to the surface being treated. A guide coat is applied to repair areas of a variety of articles such as vehicles (boats, automobiles, trucks, trains, planes, etc.), shower tubs, spas, etc. Examples of materials which have been used as guide coats on surfaces such as gel coats, putties and paints are spray paint, powders, grease pencils, and gun metal bluing compounds such as Dykem bluing compound available from Dykem Company, St. Louis, Mo., USA. These guide coats help one visualize the high and low spots during sanding. An area where guide coat remains after sanding is a low spot, whereas an area where the guide coat has been removed by sanding indicates a high spot. Thus, the surface irregularities are clearly visible to the operator, enabling the operator to abrade and fill the appropriate areas.
Guide coats are also used when sanding between grades of coated abrasives. This ensures complete sand scratch removal of the previous grade of sandpaper before continuing on to the next finer grade of paper. When the guide coat is gone from the sand scratch area, this indicates that all the previous sanding marks have been removed.
The outer skin of an article to be sanded, such as a boat, may be a gel coat which is usually a pigmented polyester resin, less commonly, epoxy or vinyl ester. Examples of other outer skins which guide coats may be used on include paints such as acrylics, polyurethanes, melamines, epoxies, etc.
Known guide coats have a number of disadvantages. For example, grease pencils can potentially smear, provide inadequate coverage, interfere with the cutting properties of the sanding article, and easily become lost or misplaced.
Spray paints can also be disadvantageous as guide coats. For example, the automotive aftermarket tends to use lacquer spray paint as a guide coat. This is an aerosol that is sprayed onto the surface and allowed to dry. The trend is to move away from aerosols for environmental reasons. The drying time is varied but does not allow the technician to sand the area for quite some time. Then, when the sanding begins, the sand paper tends to load quickly with the spray paint, forcing the technician to change sheets of sand paper more frequently. Also, the spray paint tends to float around the body shop (or other location where the sanding is taking place) and contaminate other surfaces. Depending upon where the paint lands it can be quite difficult to remove.
Gun metal bluing compounds can also be disadvantageous as guide coats. The marine industry tends to use a gun metal bluing compound (Dykem) as a guide coat which has many problems associated with it as well. The product has to be diluted (usually with acetone) before it can be used. Acetone is flammable, regulated and generally not welcome by a marine original equipment manufacturer (OEM) in the work environment. The diluted product is typically applied to a rag and wiped onto a vertical surface of the boat to be sanded with much of the product running down the surface. This bluing compound dries immediately and sanding begins. If the surface to be sanded is a porous gel coat such a guide coat cannot be removed without sanding down to the bottom level of the pores thus requiring extensive sanding with a coarser material than what was first needed. Then the area has to be sanded with increasingly finer grades of material before compounding can begin.
Kokai patent application No SHO 16-12378, published Jan. 19, 1988, discloses a touch up method for automobiles using a guide coat which is, in general, black paint.